1. Field
The field of the present invention relates generally to wireless communications and position location. More particularly, the present invention relates to apparatus and methods for position determination of wireless, mobile devices using both satellite-based positioning signals and terrestrial-based positioning signals.
2. Background
Position location has become significantly easier and more accurate since the development of positioning satellite systems. These positioning satellite systems are generally known as Global Navigational Satellite Systems (GNSS). One example of a system of positioning satellites is the U.S. NAVSTAR Global Positioning System (GPS). Another example is the GLONASS System maintained by the Russian Republic. Other positioning satellite systems being planned include the European GALILEO system. GNSS receivers are currently available for use in aircraft, ships, ground vehicles and hand-held devices for determining position location.
The various systems use multiple satellites (e.g., NAVSTAR GPS employs thirty-two satellites with twenty-four active) that orbit the earth in multiple orbital planes (e.g., NAVSTAR GPS satellites are in six orbital planes). The satellites repeat almost the same ground track as the earth turns beneath them each day. The orbital planes are equally spaced and inclined with respect to the equatorial plane, thus ensuring that a line-of-sight path exists to at least five satellites from any unobstructed point on the earth at all times.
Each satellite carries a highly accurate atomic clock that is synchronized to a common time base (GNSS time). Ground-based monitor stations measure signals from the satellites and incorporate these measurements into orbital and clock models for each satellite. Navigation data and satellite clock corrections are computed for each satellite from these models and are uploaded to each satellite. The satellite then transmits a navigation message that includes information relating to its position and its clock correction parameters.
An autonomous GNSS receiver calculates its position by combining data from the navigation message (which indicates the position of the satellite) with the measured delay of the signal received from the satellite (which indicates the position of the receiver relative to the satellite). Because of offsets in the receiver's time base relative to the GNSS time base, signals from at least four satellites are typically required to resolve a position in three dimensions and the time offset.
Problems in GNSS signal detection may occur when a GNSS receiver cannot receive a line-of-sight signal from a sufficient number of satellites. In obstructed environments (e.g., indoors, underground, obstructed locations, etc.), it may not be possible for a GNSS receiver to receive adequate (in quantity and quality) signals to make an accurate position determination.
A conventional pseudolite is a stationary terrestrial transmitter that receives one or more GNSS signals and generates and transmits a digitally coded waveform at a GNSS carrier frequency to augment the GNSS position solution. In the NAVSTAR GPS system, PRN binary sequences 33 through 37 are not assigned to satellites and may be used by a conventional pseudolite to generate and transmit a coarse acquisition (C/A) waveform. If the timing and position of a conventional pseudolite are known with high precision, then its transmitted digitally coded waveform may be used to make a position determination along with other GNSS waveforms. Thus, conventional pseudolites may be used to augment GNSS coverage.
FIG. 1 illustrates a position determination system 100 using a conventional pseudolite 150 to augment GNSS coverage. GNSS satellites 110 provide reference signals to mobile station 130 for position determination. The line of sight between one or more GNSS satellites 110 and the mobile station 130 may be blocked or impaired by obstruction 120 depending on the instantaneous locations of both the GNSS satellites 110 and the mobile station 130. An obstruction 120 may include, but is not limited to, an indoor environment, a bridge, a building, topographical features such as mountains or canyons, etc. With a line of sight blocked or impaired, the mobile station 130 may not receive the required GNSS reference signals from the GNSS satellites 110 for position determination. To overcome the obstruction problem, one typical solution uses a conventional pseudolite 150 to transmit a reference signal to the mobile station 130. The transmitted reference signal is synchronized by the conventional pseudolite 150 to GNSS timing. Conventional pseudolite 150 is a terrestrially based transmitter to augment the GNSS satellites 110. Another typical solution uses a conventional pseudolite 150 to relay at least one reference signal from the GNSS satellites 110 to mobile station 130. However, conventional pseudolites require a line-of-sight signal from one or more GNSS satellites and are useful only where a GNSS signal is available. In some cases, the line of sight between a conventional pseudolite 150 and the GNSS satellites 110 is also obstructed, thus compromising the ability of the conventional pseudolite 150 to augment the GNSS satellites 110. Additionally, a mobile station 130 may be in an underground environment, such as a subway, a basement of a building, a tunnel, etc., where reference signals from conventional pseudolite 150 cannot penetrate or is heavily impaired. In such a case (absent a long cable run from the underground environment to a surface based GNSS receiving antenna), the ability of the conventional pseudolite 150 to substitute for the GNSS satellites 110 for the mobile station 130 is degraded.
In environments where GNSS coverage is difficult, CDMA mobile system coverage may be possible. Processing of the CDMA pilot phase measurements by the mobile station may yield its position if the locations of the CDMA source signals are known. However, when repeaters are used to re-transmit the CDMA signals, this introduces ambiguities in the locations of the CDMA source signals which results in unusable CDMA pilot phase measurements for mobile station position determination. If the CDMA signals could be unambiguously identified as transmitted from particular repeaters from particular locations, then these repeated CDMA signals could be used advantageously for mobile station position determination. Accordingly, it would be desirable to provide methods and apparatus for using a CDMA Time Pseudolite to identify repeated CDMA signals unambiguously so that they may be used for position determination of a mobile station.